The combustion of fossil fuels with relatively pure oxygen has been considered for a variety of reasons, primarily related to the desire to eliminate or effectively reduce the dilution effects of the nitrogen in air. In the past, interest was in reducing the formation of nitrogen oxides during combustion. More recently, the desire to produce a concentrated stream of carbon dioxide for capture and sequestration in order to reduce greenhouse gas emissions has provided a driving impetus. Large quantities of oxygen are commercially produced via the cryogenic separation of oxygen from nitrogen in air. However, the energy requirements for this process are quite high relative to the combustion process, ranging from about 20% to about 30% of the fuel energy depending upon the oxygen purity required. This energy consumption greatly reduces the output of steam and electricity in power plants (see Bozzuto, et al., 2001, OCDO/AEP retrofit study). New technology developments seek to generate oxygen with lower energy consumption. Economic studies indicate that these approaches could improve plant economics (see “Greenhouse Gas Emissions Control By Oxygen Firing In Circulating Fluid Beds” Vol. II (Nsakala, et al., 2003). Advanced systems for the separation of oxygen from a feed gas stream include, for example, pressure swing systems, physical and chemical adsorption systems, and membrane systems. In such systems, the feed gas stream passes over a sorbent, membrane, or the like, and at least a portion of the oxygen within the feed gas is removed. In many of these systems, a high temperature sweep gas is required to provide a chemical driving force for oxygen separation or to regenerate the sorbent material. In effect, the sweep gas stream “sweeps” the oxygen away from the oxygen producing device (e.g., sorbent, membrane, or the like). To ensure the proper operation of the oxygen producing device, the sweep gas and feed gas streams must be provided to the oxygen producing device at temperatures within specified temperature ranges (see U.S. Pat. No. 6,562,104).
One example of a membrane system is the oxygen transport membrane as discussed in U.S. Pat. No. 6,406,518. In this system, air is heated before passing through a ceramic membrane. On the other side of the membrane, a gas with very low oxygen content is also preheated and passed over the outside of the membrane. The difference in oxygen partial pressure provides a driving force for the separation of oxygen from the air through the membrane. The temperature range is given as 450° C. to 1200° C. In an absorption system, the air is passed over a medium that captures oxygen at one temperature and liberates oxygen at a higher temperature. The driving force for separation may also be provided by air compression as described in U.S. Pat. No. 6,702,570. Alternatively, a material may absorb oxygen at high pressure and release oxygen at a lower pressure, as done in pressure swing systems. The goal of all of these systems is to separate oxygen from air at a lower energy penalty than cryogenic separation.
As a high temperature environment is often required in these systems, it is desired to provide a system that allows the use of these various oxygen generation systems with a boiler system to generate steam (and electricity) with reasonable efficiency, provide a concentrated carbon dioxide stream, and provide a high temperature environment necessary to allow the oxygen separation system to function optimally.